This method can be applied especially to color plasma panels displaying a large number of half-tones and having a large size (more than one meter diagonal) for television applications.
Plasma panels work on the principle of an electrical discharge in gases. They comprise two insulating slabs each bearing at least one array of electrodes and mutually demarcating a gas-filled space. The slabs are joined to each other in such a way that the arrays of electrodes are substantially orthogonal, one representing rows and the other columns. Each intersection of electrodes defines a cell to which there corresponds a small gaseous space. A given cell is turned on by the selection of two crossed electrodes to which, at a given instant, appropriate voltage are applied so that, between these electrodes, the potential difference prompts a discharge in the gas and a light emission. The cells are positioned in rows and columns.
To obtain a color panel, strips of luminophore materials that correspond to the green, red and blue colors and are excitable by ultraviolet radiation are deposited and a gas is used, emitting ultraviolet rays during the discharge. A system of barriers between the strips is used for the physical demarcation of the cells of the panel and to limit the phenomena of the diffusion of one color over another. A video pixel consists of a triplet of cells (one red, one green and one blue).
The discharges in a plasma display panel are initiated properly if the gaseous medium in which they occur is ionized. The display panels that are being developed presently for these television applications are so-called alternating plasma panels. In these panels, the electrodes that bear the slabs are insulated from the discharge gas by a dielectric layer, generally based on magnesia.
A sustaining signal formed by a succession of square-wave signals is permanently applied to all the rows. This has the effect of maintaining each cell in the state assigned to it during an addressing phase. The addressing, which consists either of the selective lighting up or the selective extinguishing of the cells of the panel, is done in sets of one or more rows and each row is scanned several times during the period of display of an image or an image cycle.
It turns out to be the case that these color plasma display panels, in part because of the nature of the gas mixture and in part because of the technology, show difficulties in the lighting up of certain cells according to a probabilistic relationship. The gaseous mixture of the color panels is generally a mixture of neon and xenon with about 10% of xenon. This mixture distributes the ionization poorly.
During the addressing phase, certain cells are not lit up when they should be lit up, or they take more time to get lit up and, during the sustaining phase, the lighting up is not done or takes place randomly or in a deferred manner. The image displayed therefore has defects.
With regard to the structure of the panel, the cells are demarcated by barriers that have a role of confinement, i.e. they are designed firstly to prevent the discharges from propagating towards the neighboring cells which should not be lit up and secondly to prevent the ultraviolet radiation created by a discharge in a given cell from exciting the luminophores of neighboring cells and generating a lack of saturation of the colors. These confinement barriers are not appropriate for the diffusion of ionization even if their height is smaller than the spacing between the two slabs and even if they extend along a single array of electrodes.
The nature of the dielectric layer in contact with the gas mixture has the specific feature of possessing a high coefficient of secondary emission assisting in the starting of the discharge but this effect is not sufficient to resolve this problem of ionization.
In monochromatic alternating plasma display panels, this problem of ionization does not arise if, all around the panel in a frame concealed from the observer, there are provided conditioning cells that are permanently lit according to specified voltage levels and a specified chronology.
Within these cells, discharges are constantly initiated, furthering the ionization of all the gas contained in the space demarcated by the slabs. These conditioning cells are efficient even if the panel is a large-sized panel. It must be remembered that, in monochromatic panels, the gas mixture is generally neon and argon with about 0.2% of argon and its role in the diffusion of the ionization is important.
The transposition of these conditioning cells outside a useful zone in a color alternating plasma panel brings practically no improvement to this problem of ionization.
There also exist direct current plasma panels in which the electrodes are in contact with the gas mixture. Each cell is partitioned off, and the problem of ionization is even more crucial. This problem has been resolved only by placing, beside each useful cell 1 that can be seen by the observer, a conditioning cell 2 masked from the observer. The lighting up of a conditioning cell 2 precedes that of a neighboring useful cell 1. One conditioning cell is generally provided for two useful cells. A cross-section of a panel of this kind is presented in FIG. 1. The two slabs bear the reference 10a, 10b. Each of them bears an array of useful electrodes 11a, 11b. Each intersection of useful electrodes 11a, 11b defines a useful cell 1. Partition walls 3 firstly separate two neighboring useful cells 1 and secondly have a brace function to ensure the efficient positioning of the slabs 10a, 10b. Each useful cell 1 is the neighbor of a conditioning cell 2. It is separated therefrom by a barrier 4 whose height is smaller, in part, than the distance between the two slabs 10a, 10b. A conditioning cell 2 is defined by the intersection of one of the electrodes 11a which is used also to define a useful cell 1 and a conditioning electrode 5.
A diagrammatic view has been shown of a conditioning discharge 6 that occurs in the conditioning cell 2 and precedes a useful discharge 7 that occurs in the useful cell 1 to the right. The conditioning discharge 6 is concealed from the observer (represented diagrammatically by an eye) for the slab 10b facing the observer carries a black matrix 8 to form a shield against the conditioning discharges 6. The conditioning discharge 6 initiates the useful discharge by pre-ionizing the gas mixture contained between the two slabs 10a, 10b.
This structure with conditioning cells requires an array of electrodes and additional electronic circuits. It gives rise to greater electrical consumption and makes a greater amount of electrical power available.
Another drawback is that the minimum pitch between two useful cells 1 separated by a conditioning cell 2 is dictated by the size of this conditioning cell 2. Space is lost.
From the viewpoint of advantages, since the conditioning discharges are masked with respect to the observer, they introduce no inconvenient luminous background that would reduce the contrast.
Another advantage is that the addressing of the conditioning cells is separated from that of the useful cells, thus making it possible to avoid using the time devoted to the addressing of the useful cells for the addressing of the conditioning cells. It must be borne in mind that the greater the number of rows of the panel the smaller should be the amount of time devoted to the processing of a row or the greater should be the number of rows processed at the same time.
Since the problem of ionization encountered in the color alternating plasma display panels is not as crucial as in panels that have direct-current type operation, it does not seem to be necessary to introduce the conditioning cells into the vicinity of each useful cell because of all the technological and electronic complications that they give rise to.
It has been proposed, in the alternating color plasma display panels described in the patent application EP-A1-0 549 275, filed by FUJITSU, to provide for a non-selective ionizing phase before each addressing phase. This means that this phase is applied simultaneously to all the rows.
FIG. 2 provides a diagrammatic view of the processing operations applied to all the rows of a display panel of this kind.
During an image cycle which is the time needed to display an image, all the rows are simultaneously ionized and then addressed and then sustained. These three phases of ionizing, addressing, and sustaining form one cycle and several cycles are repeated during an image cycle. So as to make it possible to display the half-tones, the sustaining phases of the different cycles have different durations.
This phase of ionization consists of various operations to light up all the cells of the panel, these lighting-up operations alternating with various operations of extinguishing all the cells of the panel.
In the drawing, the ionization phase is represented by hatches, the addressing phase by strokes and the sustaining phase by dots.
Apart from the fact that the cycle time is extended, these ionization phases create a relatively strong luminous background on the screen, the contrast between the lit cells and the extinguished cells being about 100.
In the color alternating plasma display panels in which the scanning operations are interlaced, it is not possible to position this non-selective ionization phase simultaneously on all the rows since the addressing and sustaining phases are temporally mixed with each other. At a given instant, not all the rows are processed in the same way.